We do. We've never had anything different so I'm not sure if they are cost effective or not. We put in £60 electric and £40-£50 gas a month (depending on the weather, in the summer, we just stock up). We have a two bedroom, one bath downstairs terraced house.
You pay a standing charge, but it's taken off the credit you put on weekly. Basically you go to the shop and top up your key/smart card and put it in your meter - that tells your meter to let the tenner's worth of power through, and registers the payment of a tenner on your account at the power company. Means that when your bill's generated (you do still get your meter read like normal - contrary to what a lot of people think) - you've built up a bank of credit on your account to cover whatever you've used... sometimes the system can fail and fall out of sync on this but it happens less with the newer meters (used to be a berluddy nightmare with the paper tokens).
Bad points are it cuts you out of things like dual fuel discounts, discounts for direct debits etc etc - so tends to be not the most competitive tarriffs available. Good points - obviously you don't get the stinkingly large bill at the end of the quarter (I also did a stint opening mail and some of the comments and material you got sent back with people's bills were... interesting to say the least - for some reason people view pictures of naked women as adequate payment for their eleccy). Bad point - sod's law dictates the time you're running low on credit is when it's piddling it down with rain on a freezing cold night and going to trek round to the local shop is the last thing you feel like doing.
Been a few years since I was on one myself - old rented flat used to have one - but I used to put something like £20 a fortnight on the eleccy there for a one-bed flat with the eternal evil known as storage heaters (only had a couple of them switched on though) - would reckon that's gone up a fair whack since then though!
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